1. Field of the Invention
This invention relates to manufacturing rigid plastic articles from continuous sheets of thermoformable plastics, such as foamed polystyrene and the like. It particularly relates to devices and methods for minimizing warping of such articles along the sides and edges thereof. It especially relates to a thermoforming technique and an improved mold design to maintain or restore the desired shape of a polystyrene foam article, such as a meat tray, during molding of the subsequent shot of articles.
2. Review of the Prior Art
Thermoformed articles are often manufactured by roll-fed thermoforming machines equipped with matched metal molds, consisting of both plugs and cavities which contain the detailed shapes that are desired on the inside and outside, respectively, of the articles being thermoformed from a continuous sheet of heat-softened plastic, such as acrylonitrile-butadiene-styrene, nylon, polycarbonate, oriented polystyrene, foamed polystyrene, polypropylene, and the like. With the exception of some minor post-mold expansion that is generally inherent with thermoformed foamed products and some overall parts shrinkage that typically occurs, it is desirable that the thermoformed part otherwise remain dimensionally consistent to the size and shape of the production mold.
When thermoformable plastics are extruded and formed into a sheet, selected amounts of cross-machine and machine-direction orientation are generally built into the sheet by stretching the extruded sheet in both directions as it cools, in order to obtain a number of desirable properties. When such as sheet is later heated to a critical temperature, it tends to shrink in both directions, thereby creating stresses in the plane of the sheet if this shrinking is resisted.
During the manufacture of plastic articles by continuous thermoforming operations on a thermoformable sheet of this type, the roll-fed sheet initially passes through an oven while its edges are being gripped by endlessly moving pins which are mounted on a pair of parallel belts or chains. Cross-machine tensile stresses in the extruded sheet are thereby relieved somewhat and utilized to maintain the sheet in horizontal posture, i.e., to prevent significant sagging of the sheet while it is passing through the oven.
However, the machine-direction orientation stresses are simply passed along to the continuously interconnected sheet passing through the thermoforming machine. These stresses are frequently manifested at the inlet end of the forming section of the machine, particularly after the molding period when the mold opens and the sheet is being indexed out of the forming station of the machine. For this reason, the rear wall of the rearmost row of articles is apt to be deformed by being pulled to a shallower angle (relative to the horizontal plane), even before the succeeding shot begins. Such distortion or deformation reduces the overall height of the molded product and creates a condition that causes inconsistent product trimming.
It is believed that certain stresses additionally occur in the heat-softened thermoplastic sheet when it is formed between the mold cavities (female) and the mold plugs (male). These stresses are balanced in the thin strips between adjacent molded articles, both in the machine direction and in the cross machine direction. However, at the edges of a roll-fed thermoforming machine, there are no opposing stresses to balance the inward stresses in the cross-machine direction created by the outermost mold plugs except for the pins of the sheet conveying mechanism along the edges of the sheet. This characteristic is particularly noticable in the machine direction at the exit end of the mold and along the rearmost edge of a shot of newly thermoformed articles.
For example, in high-speed thermoforming operations of a roll-fed thermoforming machine, the thermoformable sheet is indexed forward an amount approximately equal to the length of the molds while the molds are in open position. This indexing operation moves the finished parts of a completed sheet out of the mold area and moves a sheet of moldable material from the oven into the mold area.
As the sheet indexing operation is nearly completed, the mold sections begin to close. One mold section, containing the cavities, generally closes to the sheet line before the other mold section, containing the plugs, reaches the other surface of the sheet.
During the interval between the time that the mold cavities contact one surface of the sheet and the time that the mold plugs reach their full closed position, tensile stresses occur in the plane of the sheet as the plastic articles are thermoformed to the desired shape. Along the forward edge in the machine direction, these stresses are transmitted as stretching forces to the rearmost row of newly thermoformed articles of the previous shot that are alongside the molding machine, thereby tending to draw the rearmost edges of these existing articles into the mold area while the current mold shot is being formed. To cope with this condition, known past practice has been alternatively to provide a sheet clamping bar on the inlet (oven end) of the mold, provide a sheet clamping bar on the exit end of the mold, to pinch the sheet prior to the mold closing, or to extend the fixed mold by an additional cavity, or portion thereof, on the exit end for remolding the last row of thermoformed parts. U.S. Pat. No. 4,009,981, for example, teaches the use of clamping bars in the form of gates at both the inlet and exit ends of the mold station of the machine. Each of these techniques only partially solves the problem of part distortion, because the clamping bar does not reshape the thermoformed part if the part has already become distorted and because the overlap thermoforming technique allows the mold, as it is forming the new shot, to stress the rear side of the last row of parts before the overlap section of the mold fully contacts the previously thermoformed parts.
There is consequently a need for a device and method that will: (a) obviate transmittal of these in-sheet stresses from the molding machine to the last row of newly formed thermoformed articles, (b) preserve the shape of the newly thermoformed articles along this last row, and (c) if necessary, even reshape these newly thermoformed articles if distortion has developed such as through the indexing operations or from contracting forces in the thermoformable sheet during the molding operation, due to machine-direction orientation.